Progesterone (P) plays a key role in the pathophysiology of "catamenial epilepsy", a menstrual cyclerelated seizure disorder that affects many women with epilepsy. While P's endocrine actions are mediated by the progesterone receptor (PR), P's molecular mechanism of action in seizure activity is not clearly understood. P modulates seizure activity partly through conversion to allopregnanolone, which enhances GABAA receptor function. Recently, we have made the novel discovery that PRs play a key role in seizure susceptibility. Our preliminary results underscore that PR knockout (PRKO) mice exhibit elevated seizure threshold and resistance to kindling epileptogenesis, suggesting that PRs mediate seizure susceptibility. In this application, we propose to elucidate the molecular mechanisms whereby PRs increase seizure susceptibility in the hippocampus, utilizing genetic, molecular and electrophysiological approaches. We hypothesize that progesterone's actions on seizure activity are due in part to PR regulation of hippocampal GABA^ receptor subunit expression and function that results in reduced inhibition and thereby seizure susceptibility. PR-mediated "subunit switching" leading to alterations of GABAA receptor subunit composition and functional properties are the molecular mechanism by which P controls seizure susceptibility. We propose to test this hypothesis critically utilizing 3 model systems that are well established in our lab: 1) PRKO transgenic mouse model;2) Antisense PR inhibition in wild-type mice;and 3) RU-486 blockade of PR function in wild-type mice. The Specific Aims of this proposal are: 1) to determine whether PRs increase seizure susceptibility using the kindling model of epilepsy;and 2) to determine whether PRs are involved in P induced alterations in GABAA receptor subunit expression and channel function. Aim 1 will examine the rate of hippocampus kindling in WT and PRKO mice, with and without P and/or finasteride treatment, as an indicator of epileptogenesis. Aim 2 will assess mRNA (real-time PCR) and protein levels (Westerns blots) of GABAA receptor subunits in hippocampal subfields in WT and PRKO mice treated with P, P+finasteride, and 24 hrs after neurosteroid withdrawal. We will verify the functional importance of P induced changes in GABAA receptor subunit expression by recording GABA-gated CI" currents in acutely isolated hippocampal neurons using patch-clamp electrophysiology. Significance. The proposed study will provide novel information on the P mechanisms governing regulation of seizure susceptibility, which could lead to improved understanding of catamenial epilepsy. Relevance. Women with catamenial epilepsy have seizures clustered around their monthly cycle. However, currently the pathophysiology of this condition is not fully understood. The studies proposed in this application will help better understand the disease mechanism and thereby allow development of specific therapies for catamenial epilepsy and other conditions associated with fluctuations in progesterone levels.